City directories can be very useful records for genealogists tracing their urban ancestors. Most of us have looked up at least one ancestor in a directory, but many do not go beyond this. Yet researchers can glean much more from the directories by making full use of their features.

Identify the ward for census research. Many people have common surnames that make identifying ancestors in census records difficult. Discerning your John Sullivan ancestor among the many others, without having to look up every family, will save time and effort. Street directories can identify the streets and wards for heads-of-households.
Identify the street address before 1880. City directories can provide the street addresses for pre-1880 censuses. In many of the cities, the 1880 and later censuses recorded the street address, but not so for the pre-1880 censuses. By looking at the 1850, 1860 or 1870 street directories, genealogists can determine exactly where their ancestor lived. African Americans were frequently listed separately at the end of the main directories.
Follow families annually. Many immigrants moved around frequently as their families changed size (or as they were evicted for non-payment of rent). Once you have found your ancestor in a federal or state census, you can follow him forward in time between censuses, on an annual basis, in large cities. In addition, directories listed both address of employment and residence, data that allow you to identify your ancestor among others with the same name. Some directories will also list the new town of residence when your ancestor moves out.
Research the neighbors. Sometimes the naturalization records do not provide enough detail to determine your ancestor's origin. Since many immigrants clustered in neighborhoods comprised of people from the old country, you can trace the origin of your ancestor by proxy, i.e., by researching their neighbors' origins. Twentieth century directories sometimes have "criss-cross" directories in the back section. These features list the heads-of-households by street, and help determine the names and addresses of the neighbors.
Find a renumbered address. Even when you find your ancestor's street address in a directory, you may discover that the streets have been renumbered over time. Many directories feature listings of street numbers and cross streets, so that you can determine on which city block your ancestor lived.
Find the local church. Researching in church registers can provide crucial information about our ancestors. Many directories provide not only the names and addresses of residents, but also listings of municipal offices, businesses, social organizations, newspapers and churches.
Identify the priests or ministers in marriage records. The informants for civil marriage records are often the priests or ministers who performed the marriages. Directories may list the street address of the priest or minister, and sometimes list all the personnel by church. With this information, you can look for the church record of the marriage, and possibly subsequent baptisms.
Find the date of death. Finding a civil death record or an obituary can be daunting for an ancestor with a common name. By following your ancestor in the directory until he disappears, or a widow appears at the same address, you can determine the possible year of death. Furthermore, many twentieth century directories provide the exact dates of death.
Identify and locate others with same surname. Perhaps you have struck out finding a useful naturalization for your ancestor. You can identify others with the same surname, look for possible connections with your ancestor and research their origins. This strategy works best for less common surnames.
Look for others with the name surname at same address or next door. As your ancestor's children matured and obtained employment, they may have appeared in street directories as individuals living at the same address. This feature is particular1y helpful in the period between 1880 and 1900, since the 1890 census was destroyed. A child could have been born in 1880 and have moved away by 1900. Directories can help establish a relationship between the parent and child when other records are missing.